In present audio radio communication systems used onboard aircraft, a certain percentage of time is spent by the pilot or co-pilot determining whether a communication link has been established with an intended receiver. This time, called overhead time, not only prevents the pilot from concentrating on other tasks related to flying the aircraft but also contributes to the inefficient use of radio communications frequencies. In a typical aircraft radio communication system, the overhead time is the time it takes the pilot to transmit a voiced call signal to a destination receiver asking the destination receiver to respond plus the time it takes to receive a voiced acknowledge signal from the destination receiver indicating that the call signal was received. The pilot, upon hearing the received acknowledge signal, knows that a communication link has been established with the destination receiver and that the pilot can then begin transmitting information that will be received by the destination.
While such a method of establishing a communication link works well for communication systems having no need to limit overhead time, there are some situations in which it is desirable to (a) eliminate the need for an operator to spend time transmitting signals in order to determine whether a communication link has been established and (b) reduce the time spent transmitting signals having little informational value (like the call and acknowledge signals). This is particularly true in a networked communication system where one operator may be attempting to communicate with several destination receivers at the same time. At the present time, an operator communicating on a network must determine if he is in communication with each network receiver by transmitting a call signal to all of the network receivers and waiting for an acknowledge signal to be transmitted from each receiver that is in communication with the operator. This process not only involves multiple transmissions of signals having little informational value, thereby contributing to an inefficient use of radio communication frequencies, but is also distracting for the operator.
Therefore, a need exists for a communication system that automatically provides an operator with an indication of whether a communication link has been established with an intended destination receiver. Such a communication system should reduce the overhead time spent determining whether a communication link has been established, as well as increase the efficiency of a radio frequency by minimizing the amount of time that radio signals are transmitted and received on the particular frequency for the purpose of determining if a communication link has been established.